1. Technical Field
The present invention generally relates to a method for improving copper corrosion performance of a lubricating oil composition.
2. Description of the Related Art
Lubricating oil compositions used to lubricate internal combustion engines and transmissions contain a major amount of a base oil of lubricating viscosity, or a mixture of such oils, and one or more lubricating oil additives to improve the performance characteristics of the oil. For example, lubricating oil additives are used to improve detergency, to reduce engine wear, to provide stability against heat and oxidation, to reduce oil consumption, to inhibit corrosion, to act as a dispersant, and to reduce friction loss. Some additives provide multiple benefits such as, for example dispersant-viscosity modifiers.
Among the most important additives are dispersants which, as their name indicates, are used to provide engine cleanliness and to keep, for example, carbonate residues, carboxylate residues, carbonyl residues, soot, etc., in suspension. The most widely used dispersants today are products of the reaction of succinic anhydrides substituted in alpha position by an alkyl chain of polyisobutylene (PIBSA) type with a polyalkylene amine, optionally post-treated with a boron derivative, ethylene carbonate and the like.
Among the polyamines used, polyalkylene-amines are preferred, such as diethylene triamine (DETA), triethylene tetramine (TETA), tetraethylene pentamine (TEPA), pentaethylene hexamine (PEHA) and heavier poly-alkylene-amines (HPA).
These polyalkylene amines react with the succinic anhydrides substituted by alkyl groups of polyisobutylene (PIBSA) type to produce, according to the molar ratio of these two reagents, mono-succinimides, bis-succinimides or mixtures of mono- and bis-succinimides
Such reaction products, optionally post-treated, generally have a non-zero basic nitrogen content of the order of 5 to 50, as measured by the total base number or TBN, expressed as mg of KOH per gram of sample, which enables them to protect the metallic parts of an engine while in service from corrosion by acidic components originating from the oxidation of the lubricating oil or the fuel, while keeping the said oxidation products dispersed in the lubricating oil to prevent their agglomeration and their deposition onto metal parts.
Dispersants of mono-succinimide or bis-succinimide type are even more effective if their relative basic nitrogen content is high, i.e. in so far as the number of nitrogen atoms of the polyamine is larger than the number of succinic anhydride groups substituted by a polyisobutenyl group.
However, these dispersants such as succinimide dispersants are also known to cause some corrosion of heavy metal bearings, for example, copper and lead components. However, before certifying a crankcase lubricant for use in their engines, engine manufacturers (oftentimes referred to as “original equipment manufacturers or “OEMs”) require passage of a number of performance tests, including a copper corrosion test.
Therefore, it would be desirable to develop lubricating oil compositions which exhibit improved copper corrosion performance.